Light emitting diodes (LEDs) generate light in zones so small (a few mm across) that it is a perennial challenge to spread their flux uniformly over a large target zone, especially one that is much wider than its distance from the LED. So-called short-throw lighting, of close targets, is the polar opposite of spot lighting, which aims at distant targets. Just as LEDs by themselves cannot produce a spotlight beam, and so need collimating lenses, they are equally unsuitable for wide-angle illumination as well, and so need illumination lenses to do the job.
A prime example of short throw lighting is the optical lens for the back light unit (BLU) for a direct-view liquid crystal display (LCD) TVs. Here the overall thickness of the BLU is usually 26 mm or less and the inter-distance between LEDs is about 200 mm. Prior art for LCD backlighting consisted of fluorescent tubes arrayed around the edge of a transparent waveguide, that inject their light into the waveguide, which performs the actual backlighting by uniform ejection. While fluorescent tubes are necessarily on the backlight perimeter due to their thickness, light-emitting diodes are so much smaller that they can be placed directly behind the LCD display, (so called “direct-view backlight”), but their punctuate nature makes uniformity more difficult, prompting a wide range of prior art over the last twenty years. Not all of this art, however, was suitable for ultra-thin displays.
Another striking application with nearly as restrictive an aspect ratio is that of reach-in refrigerator cabinets. Commercial refrigerator cabinets for retail trade commonly have glass doors with lighting means installed behind the door hinging posts, which in the trade are called mullions. Until recent times, tubular fluorescent lamps have been the only means of shelf lighting, in spite of how cold conditions negatively affect their luminosity and lifetime. Also, fluorescent lamps produce a very non-uniform lighting pattern on the cabinet shelves. Light-emitting diodes, however, are favored by cold conditions and are much smaller than fluorescent tubes, which allow for illumination lenses to be employed to provide a much more uniform pattern than fluorescent tubes ever could. Because fluorescent tubes radiate in all directions instead of just upon the shelves, much of their light is wasted. With the proper illumination lenses, however, LEDs can be much more efficient, allowing lower power levels than fluorescent tubes, in spite of the latter's good efficacy.
The prior art of LED illumination lenses can be classified into three groups, according to how many LEDs are used:    (1) Extruded linear lenses with a line of small closely spaced LEDs, particularly U.S. Pat. Nos. 7,273,299 and 7,731,395, both by these Inventors, as well as References therein.    (2) A line of a dozen or more circularly symmetric illumination lenses, such as those commercially available from the Efficient Light Corporation.    (3) A line of a half-dozen (or fewer) free-form illumination lenses with rectangular patterns, such as U.S. Pat. No. 7,674,019 by these Inventors.
The first two approaches necessarily require many LEDs in order to achieve reasonable uniformity, but recent trends in LEDs have produced such high luminosity that fewer LEDs are needed, allowing significant power savings. This is the advantage of the last approach, but free-form lenses generating rectangular patterns have proved difficult to produce, via injection molding, with sufficient figural accuracy for their overlaps to be caustic-free. (Caustics are conspicuous small regions of elevated illuminance.)
What is needed instead is a circularly symmetric illumination lens that can be used in small numbers (such as five or six per mullion) and still attain uniformity, because the individual patterns are such that those few will add up to caustic-free uniformity. The objective of this Invention is to provide a lens with a circular illumination pattern that multiples of which will add up to uniformity across a rectangle. It is a further objective to attain a smaller lens size than the above mentioned approaches, leading to device compactness that results in lower manufacturing cost. The smaller lens size can be achieved by a specific tailoring of its individual illumination pattern. This pattern is an optimal annulus with a specific fall-off that enables the twelve patterns to add up to uniformity between the two illuminating mullions upon which each row of six illuminators are mounted. This fall-off at the most oblique directions is important, because this is what determines overall lens size. The alternative approaches are: (1) Each mullion illuminates 100% to mid-shelf and zero beyond, which leads to the aforementioned caustics; (2) Each mullion contributes 50% at the mid-point, falling off beyond it. The latter is the approach of this Invention, and has proven highly successful.
The prior art is even more challenged, moreover, when fewer LEDs are needed due to ongoing year-over-year improvements in LED flux output. After all, backlight thickness is actually relative to the inter-LED spacing, not to the overall width of the entire backlight. For example, in a 1″ thick LCD backlight with 4″ spacing between LEDs, the lens task is proportionally similar to the abovementioned refrigerator cabinet. Because of the smaller size of an LCD as compared to a 2.5 by 5 foot refrigerator door, lower-power LEDs with smaller emission area will be used, typically a Top-LED configuration with no dome-like silicone lens.
Regarding the prior art patents which have taught non-specific design methods for addressing this problem are: US 2006/0138437, U.S. Pat. No. 7,348,723, U.S. Pat. No. 7,445,370, U.S. Pat. Nos. 7,621,657 and 7,798,679 by Kokubo et al. shows the same cross-sectional lens profile as in FIG. 15A of U.S. Pat. No. 7,618,162 by Parkyn and Pelka, while failing to reference it. U.S. Pat. No. 7,798,679 furthermore contains only generically vague descriptions of that lens profile, and worse yet has no specific method of distinguishing the vast number of significantly different shapes fitting its vague verbiage, its many repetitively generic paragraphs notwithstanding. Experience has shown that illumination lenses are unforgiving of small shape errors, such as result from unskilled injection molding or subtle design flaws. Very small changes in local slope of a lens can result in highly visible illumination artifacts sufficient to ruin an attempt at a product. Therefore such generic descriptions are insufficient for practical use, because even the most erroneous and ill-performing lens fulfills them just as well as an accurate, high-performing lens. Thus U.S. Pat. No. 7,798,679 does not pertain to the preferred embodiments disclosed herein, because it never provides the specific, distinguishing shape-specifications whose precise details are so necessary for modern optical manufacturing.